1. Field of the Invention
This invention relates to differential pressure transducers, and more particularly, to differential pressure sensors having a digital output and a low sensitivity to common mode line pressure errors.
2. Description of the Prior Art
A servo torque-balance pressure transducer with low sensitivity to common mode line pressure errors is described in U.S. Pat. No. 3,664,237 issued to Paros. This analog output sensor achieves low sensitivity to common mode line pressure errors through delicate adjustment of lever arm distances between the two bellows pressure inputs and a flexible pivot. The lever arm adjustment is designed to equalize the moments produced about the pivot axis when the same line pressure is applied to both bellows. Differential pressure between the bellows produces a resultant torque which may be measured using conventional servo techniques. The adjustment mechanism is complex, difficult to manufacture and requires inordinate skill to reduce the common mode line pressure errors to acceptable levels. One difficulty in this design is that since the full scale differential pressure range is determined by the effective areas of the bellows and the lever arm distances to the pivot, adjustments to the lever arm distances are directly proportional to full scale and, therefore, must be made with great precision. Common mode line pressure errors can nevertheless be reduced to an acceptable level with this device, since it produces an analog output which is inherently limited in accuracy. However, an analog output is less desirable than a direct digital-type signal because a digital signal is more compatible with digital computers and control systems as well as inherently more accurate. But it is this greater accuracy which makes common mode line pressure errors more apparent than they would be with a device producing a less accurate analog output. Thus, conventional differential pressure mechanisms are inherently incapable of providing optimum performance when used with a digital sensor since their common mode pressure error are an unacceptably high percentage of their differential pressure sensitivity.
In a paper delivered at the 27th Annual ISA Conference and Exhibit in October, 1972, and published in ISA Transactions, Vol. 12, 1973, pp. 173-1979, Paros describes the design of a "Precision Digital Pressure Transducer." This sensor employs two coaxial opposing bellows, with sealed ends attached to a pivotal suspension system. The bellows are mounted with the open ends adapted to receive pressure inputs such that differential pressure between the two bellows causes a resultant torque which stresses a vibrating quartz crystal whose frequency change is a measure of the differential pressure. No provision for reducing the common mode line pressure errors is described. These errors are caused by differences between the bellows effective areas or lever arm distances to the pivot and are directly proportional to full scale.
A number of load sensitive resonators are known. In an unstressed state, under constant environmental conditions, a vibrating element has a unique resonant frequency determined by its dimensions and material composition. This resonant frequency increases under tensile loading and decreases under compressive loading. The resonant frequency should be a true and accurate measure of the applied load. For optimum performance, stress-sensitive resonators should have a well-defined resonant frequency. Energy losses from the resonator reduce the "Q" or quality factor of the resonator thereby making the resonant frequency less well defined. One cause of energy loss from a resonator is friction between the resonator and the surrounding environment. The surrounding environment can also, with time, alter the performance of the resonator thus further degrading its performance. Consequently, it is desirable for resonators to operate in a vacuum or inert atmosphere.
Force sensitive crystal resonators are described in U.S. Pat. No. 2,984,111 issued to Kritz and U.S. Pat. No. 3,093,760 issued to Tarasevich in which loads are applied to crystals near the nodal points.
U.S. Pat. No. 3,470,400 issued to Weisbord describes a single beam force transducer with an integral mounting system which effectively decouples the beam vibrations from the mounting points through a spring and mass arrangement.
U.S. Pat. No. 3,238,789 issued to Erdley describes a load sensitive closed end tuning fork consisting of two tines or bars vibrating 180 degrees out of phase such that the reactive forces and moments cancel.